Recent advances in the ability of seismic data acquisition and processing equipment to handle large amounts of data have stimulated greater interest in the development of versatile high energy seismic sources adapted to produce both compensational and shear waves. One goal in the development of "multi-component" seismic sources is to enable the operator to more efficiently measure sub-surface formation properties such as porosity, fracture orientation, pore fluid type, or lithologic type. It is known that P, SH and SV waves travel with their own distinct velocities when passing through a particular geologic formation. Thus, changes in formation property, such as rock type, porosity and the like, may be detectable by changes in the full wave field (i.e., the P, SV and SH wave velocities). A strong correlation observed between lateral variations in the compressional to shear wave velocity ratio (Vp/Vs), with known changes in the geologic formation, suggests that these variations may be used to infer changes in the lithology, porosity, or pore fluid content of the sub-surface formation. By recording the full waveform of a seismic wave, including P, SH and SV waves, one can get far more detailed information concerning rock properties at depths than would otherwise be possible.
There are two major types of surface seismic sources currently available which have been adapted to the production of combined P, SH, and SV waves. One is the impulsive-type source wherein a heavy mass is propelled in a single downward stroke to impact a ground engaging target. The other is the reciprocating vibratory-type transducer which imparts a controlled, varying frequency signal into the earth, as exemplified by the Vibroseis.RTM. reciprocating vibrator seismic source (a registered trademark of Conoco, Inc.). It is with the latter type of seismic source that the invention to be described is particularly concerned.
The general concept of simultaneously imparting both compressional and shear wave vibrational energy to the earth with seismic exploration apparatus is taught in U.S. Pat. No. 4,321,981 issued to K. H. Waters on Mar. 30, 1982, entitled "Combination Shear Wave and Compressional Wave Seismic Energy Vibrator." In accordance with the above referenced patent, a combination of two or more vibrating masses may be operated along opposing non-vertical vibrational axes, oriented at a fixed angle to each other in parallel planes. The times of passage of the masses through a common central point as they vibrate along their respective axes can be controlled to establish a phase relation between the two vibratory motions. By varying the related phases of the two masses, one can effect desired inputs of compressional and shear waves simultaneously.
The referenced Waters patent teaches that the parallel planes in which the respective vibration axes of the two masses lie are to be oriented either transversely to the line of seismic survey, i.e., the line between the location of the seismic source and the recording geophones, or along such survey line. With proper phasing, the above-referenced transverse axial orientation of the vibrating masses enables one to produce and record simultaneously P waves and shear SH waves. On the other hand, if the vibrational axes are aligned with the survey line, one can generate and record simultaneously P waves and shear SV waves. The vertical component of motion in both cases produces so called "Converted" shear SV waves, but such waves cannot be processed and interpreted with conventional seismic methods. Thus, Waters teaches in effect that only by operating the patented apparatus successively in these two modes can one generate and record at a single remote location all forms of a seismic wave, including P, SH and SV waves, in a manner which permits conventional processing.
In the field of seismic exploration today, data acquisition and processing equipment has been developed to handle large amounts of data generated by apparatus constructed to produce both compressional and shear waves.
Patents illustrating full wave field vibratory seismic sources include U.S. Pat. Nos. 4,662,473, issued May 5, 1987, to Robert M. Betz; 4,655,314 issued Apr. 7, 1987, to Tom P. Airhart; 4,660,675, issued Apr. 28, 1987, to Tom P. Airhart; and 4,719,607 issued Jan. 12, 1988 to Tom P. Airhart. Each of these patents illustrates apparatus capable of producing vibratory seismic waves at varying inclinations and azimuths without repositioning the ground engaging base plate of the apparatus or without repositioning the vehicle utilized for carrying such apparatus.
U.S. Pat. No. 4,719,607 illustrates a general design for a multi-component vibratory seismic apparatus. This invention described hereinafter is an improvement to that apparatus.
When conducting the seismic exploration of an area of interest, it is generally necessary that the vibratory seismic source be moved to a large number of locations. Accordingly, such apparatus is usually mounted on vehicles. These vehicles must include means for raising and lowering the ground engaging base plate and the vibratory source. They also usually include means for powering the vibratory source. An example of a vehicle mounted seismic source is illustrated in U.S. Pat. No. 4,660,674 issued Apr. 28, 1987, to Tom P. Airhart. It will be noted in that patent that the seismic source is not a vibratory seismic source, but rather utilizes a heavy mass that is propelled downwardly against the surface of the base plate to produce an instantaneous heavy shock wave thereto. Despite the differences in the type of seismic source illustrated, the patent does illustrate the mounting of a source on a vehicle such as is contemplated by this invention.
From the listed patents, it can be seen that a number of attempts have been made to control the inclination and azimuth of the vibratory source relative to the base plate. While the foregoing systems work satisfactorily, it would be highly desirable to more precisely, and in a shock isolated arrangement, control the inclination and the azimuth of the seismic source. Accordingly, an object of this invention is to provide improved shock isolated seismic apparatus for imparting seismic vibratory signals to the earth at more precise inclinations and azimuths.